Is there any particular reason why one wouldn't upload both solo and combo versions and let the reader decide which one they want to pick up?

IMHO, offering combo versions has several advantages:
- similar filesize as Calibre generated .azw3/.epub (see my other post for details)
- just one file to upload/download
- backwards compatibility
- no need to set up a separate KF8/azw3 sub-forum

Quote:

Originally Posted by ATDrake

(Maybe it takes up more space on MR's servers, but it probably also saves them some infinitesimal bandwidth if people download just the stripped versions.)

In my admittedly not very scientific test, the KF8-only file generated by Calibre was only 60KB less than the Huffdic compressed combo file generated with KindleGen. (I got similar results with other files.)